It is known from the prior art (Aflal et al., Appl. Polym. Sci. 2009, 113, 2191) to repair an article based on epoxy resin. The solution proposed consisted in only partially reacting the epoxy functional groups during the manufacture of the article. This was able to be performed by using a sub-stoichiometric amount of hardener. To repair a damaged article, a high temperature is then applied to the part of the article concerned such that the epoxy functions that have remained free react together and form covalent bonds.
Another method known from patent application WO 02/064 653 for repairing a polymer-based article consists in dispersing in the polymer microcapsules filled with a polymerisable agent. Damage of the article brings about rupture of the microcapsules and the release of the polymerisable agent into the fracture. The polymerisation of this agent allows the fracture to be repaired.
However, these methods are limited to the repair of articles and cannot envisage the recycling of thermosetting resins or their transformation, once hardened, into an article having another shape. In addition, these repair methods allow the article to be repaired a maximum of only once or twice. Specifically, when all the epoxy functions have reacted—or when the polymerisable agents have polymerised—it is no longer possible to repair the component or the material. Finally, materials comprising capsules usually have inferior mechanical properties to those of the resins of which they are composed.
Polymeric systems using reversible covalent bonds have already been described. Thus, Lehn, J. M., Progress Polym. Sci., 2005, 30, 814-831 and Skene W. G., Lehn, J. M., P.N.A.S. 2004, 22, 8270-8275 disclose polymeric resins that are capable of depolymerising and of repolymerising under the action of heat. The team of Professor Wudl (Chen X. et al., Science 2002, 295, 1698-1702) has described self-repairing materials based on the reversibility of the Diels-Alder reaction.
However, these studies concern only the repair and assembly of components and do not envisage the transformation of an article based on thermosetting resin into an article of a different shape.
The document J. O. Outwater, D. G. Gerry, J. Adhesion, vol. 1, 1969, 290-298 mentions the possibility of heat-repairing a fracture in an epoxy resin. It is taught in that document that the energy restitution associated with the disappearance of the fracture surfaces is responsible for this phenomenon. However, these observations have not been repeated in more than 40 years and have not led to any development. Furthermore, the resin composition that was used in that document does not correspond to the definition of the compositions of the invention and does not make it possible to transform an article or to be subjected to recycling.
Document U.S. Pat. No. 3,624,032 describes an epoxy resin composition comprising an epoxy resin, a hardener of acid anhydride type and a catalyst of acetylacetone metallic complex type. It is noted in the examples that the components are used in solid form and the working conditions described do not make it possible to dissolve the catalyst in large amount in the thermosetting resin precursor.
Document U.S. Pat. No. 3,932,343 describes an adhesive epoxy resin composition comprising an epoxy resin, an acid anhydride and a catalyst of acetylacetone metallic complex type. It is noted that the molar amounts of catalyst are less than 1.5% relative to the number of moles of epoxy functions of the resin.
According to the present invention, thermosetting resins are endowed with chemical reversibility, which, when combined with a mechanical constraint, may be used to give an article a new shape.
Furthermore, the resin compositions of the invention are also distinguished from those of the prior art in that they are not special resins but are composed of an ordinary thermosetting resin, in particular an ordinary epoxy resin, an acid anhydride hardener capable of reacting with epoxide functions generating hydroxyl and ester functions, a standard esterification catalyst and an identical or different transesterification catalyst. They differ from standard epoxy resins by the presence of amounts of transesterification catalyst higher than those usually used, since transesterification is not usually desired or envisaged.
These compositions and the processes of the invention may thus be used in all the usual applications of thermosetting resins, in particular epoxy resins, but have the advantageous properties that have been mentioned above and are illustrated in detail in the description and the implementation examples.